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G8MNY  > TECH     11.07.18 10:03l 183 Lines 8410 Bytes #999 (0) @ WW
BID : 28639_GB7CIP
Subj: Dip Oscillator
Path: HB9ON<IW2OHX<IW0QNL<JH4XSY<JE7YGF<GB7CIP
Sent: 180711/0742Z @:GB7CIP.#32.GBR.EURO #:28639 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                     (New Nov 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is the circuit of a 1 FET dip oscillator that was used for a club on the
evening construction contest. (by G4VTD)

The "Grid Dip Oscillators" were originally valve & the dip in the current
indicated that some of the oscillator power is being "sucked away" by a nearby
resonant circuit, aerials etc. thus indicating the correct resonance or not. It
also works just as a simple signal source, but not too stable! It can even be
use for testing Crystals as poor ones won't oscillate.

This is the FET equivalent of the valve circuit.

                             ÚÄÄ1kÄÄÄÂÄÄÄ68ÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄo +9V
                             ³       ³               ³      ³
    4mm            n1       |³|fb    ³               ³      ³
       0ÄÄÄÄÄÄÂÄÄÄÄÄ´ÃÄÄÄÄÄÄÄ´      ===             5k6     ³
       ³      ³_             ³     1n³               ³      ³  o = push in post
       ³  145p³/| Small      ³       ³               ³      ³
Plug   )     ===  Ganged     ³       ÀÄo RF to       ³      ³
 in    )     /³   MW+VHF     ³          counter      ³      ³
search ) ÚÄÄÄÄ´_  Caps       ³                      Zero    ³       .-.
coils  ) ³145p³/|            ³                 ÚÄÄÄ´1k    ÚÄÁÄ¿    /...\
  L    ³ ³   ===       2N3819³                 ³    pot   ³1n ³    ÀÄÄÄÙ
       ³ ³   /³   n1     g ÃÄÙd                ³     ³   === ===   d g s
       0Ä)ÄÄÄÄÁÄ´ÃÄÂÄÄÄÂÄÄ>ÃÄ¿s          _     ³    _³_   ³ u1³    2N3819
    4mm  ³         ³   ³     ³  1n ft  /~ ~\   ³  D2\_/   ÀÄÂÄÙ
         ³        _³_ 120k ÚÄÁÄÂÄÄÄÄÄo´MeterÃoÄ´    _³_2 x  ³
         ³     D1 /_\  ³   ³   ³ ~³~  -\_ _/+  ³  D3\_/1N4  ³
         ³   1N4148³   ³  ===  1k ³      ~    ===    ³ 148  ³
         ³         ³   ³ u1³   ³  ³         u1 ³     ³      ³
         ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄ 0V

HOW IT WORKS
The split tuning caps with the search coil L gives 180ø phase reversal between
the FET's Drain & Gate to from a Colpitts oscillator. As the gate is not a
Valve grid, a diode D1 is needed to keep the gate voltages all negative. The
source voltage follows the mean Gate voltage plus the FETs bias potential
(~2V), & this appears across the source 1k resistor. The meter compares this
voltage to zero pot preset voltage.

Now if the tuned circuit gain "Q" is altered due to loss from a nearby tuned
circuit, the oscillator power will change, reducing the mean -ve Gate voltage,
so the source will go more positive & the meter will show this as you tune
across the nearby circuits resonance.

The frequency counter output is a small tapping of the drain output, set here
at 50ê Z with a 68ê for 120mV output (60mV term), for 75ê use 82/100ê for 200mV
output (100mV term).

The Ferrite Bead (fb) on the drain 1k gives a little bit more gain at VHF.

TIN CAN CONSTRUCTION
A pre punched U tin plate is used to house all components.

                 fold           fold
     ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
Holes³   .......   |       m-  +  |         ..  ³
for  ³   :O   O:   |     ft>O  O  |        (  ) ³ Large
dual ³   :  _  :   |              |         ~~  ³ Holes
gang ³  () (_) ()  |              |  0<Pot      ³ for 4mm
Cap  ³   :     :   |              |  hole   ..  ³ Banana
     ³   :O...O:   |        O  O  |        (  ) ³ Sockets
     ³             |      m+  rf  |         ~~  ³
     ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                       holes for
                 3 through push in posts,
                 1 preset adjustment pot,
                 1 feed through cap.

COMPONENT MOUNTING
All were standard small components as the design are ugly mounted & the lead
wires used to support the components on the available fixed points. The wire
connection push in post use the component leads as the connection wire. The
feed through capacitor is soldered into the Tin Can. All wire lengths kept at a
minimum in the best VHF practice.

The tuning capacitor is only soldered in, when most of the smaller components
are fitted, there are vertical wires needed on the outside of the tin can
linking its VHF & MW sections.

Finally the search coil sockets are mounted & wired up with the n1 capacitor
leads to the tuning capacitor.

LAYOUT
There was no specific layout, but the finished item looked like this.

               Banana sockets    .-.
        ³       _____ÚÅÄÄÂÄ¿ ___/   \ Search
        ³___  =[_____³³  ³ ³<_  \   / Coil
        ´   ³]    nutÀÅÄÄÁÄÙ     '-'
  Knob__³MW+³ n1      ³ ^tube
     ³__³VHF³         ³
    Cap ³Cap³ fet     ³pot
   link ´___³]        ³hole
vertical³      ÚÁ¿ ÚÁ¿³
        ÀÄÄÄÄÄÄÅÄÅÄÅÄÅÙ
               ~³~ ~³~
              External
             Connections
TESTING
With about 20cm of thick 1mm enamelled copper wire 8 turns 10mm dia as the
search coil & ends prepared to make firm connections (bent into U), the
oscillator ran between 24-70MHz with a frequency counter connected. The
indicating dip meter can be any sensitive (100uA) VU type or a centre zero
meter, or even a DVM. Once the preset Zero pot for a particular coil it should
not need adjusting.

Various coils can be
made to get the desired            ÚÄÄ((()
frequency needed, or               / ÚÄÄÄÙ
even just made up as        <_____/  / Awkward
needed for best size,               / access
access shape &              <______/ search coil
frequency range

Turns     Dia    Wire   Usable range       _         _       _____
Single U  10mm   2mm     100-200MHz      /~ ~\     /~ ~\    ݱ±±±±Þ
  2t      10mm   2mm      68-170MHz     ³     ³   ³\_ _/³   ݱ±±±±Þ
  8t      10mm   1mm      24-72MHz      ³     ³   ³  ~  ³   ³~~~~~³
 28t      10mm   0.8       8-24MHz      ³     ³   ³     ³   ³     ³
 48t      10mm   0.4       3-19MHz      U     U   U     U   U     U
110t      10mm   0.2       2- 8MHz

Heat glue melted on the coils (soldering iron) will help to keep the smaller
coils stable. With more turns & thinner wire you need a paper former (sticky
label inside out making a tube on a pen) wind the coil & again use heat glue to
hold in place. Double up or use thicker wire for the in & out leads. Much lower
frequencies can be obtained with a ferrite rod cored coil of needed for say
1.8MHz or 465kHz.

If the coils are well made & stable, a frequency scale for the capacitor can
be made for each coil.

FINISHING OFF
If all works well & you want to box all the bits, try adding a cross shape of
Tin Can....
                             ÚÄÄÄÄÄÄÄÄÄ¿
                            /³         ³\
                           ³ ³   PP3   ³ ³
                           ³ ³   batt  ³ ³short sides long
                           ³ ³         ³ ³enough for
                           ³ ³         ³ ³battery & meter
                           ³ ³         ³ ³
                           ³ ³         ³ ³
                            \³         ³/
  ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙúúúúúúúúúÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
 /³                          :         :                          :         ³
³ ³                          :         :                          :         ³
³ ³                          :         :                          :   top   ³
³ ³                          :   bnc _ :                          :         ³
 \³                          :      (_):                          :         ³
  ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿úúúúúúúúúÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                            /³         ³\
                           ³ ³         ³ ³
                           ³ ³    on   ³ ³
                           ³ ³   off[] ³ ³    line sides long
                           ³ ³         ³ ³    enough to reach
                           ³ ³  meter  ³ ³    the top of unit
                           ³ ³ÚÄÄÄÄÄÄÄ¿³ ³
                           ³ ³ÀÄÄÄÄÄÄÄÙ³ ³
                            \³         ³/
                             ÀÄÄÄÄÄÄÄÄÄÙ

       _____________________________
Search³ÚÄÄÄÄÄÄÄ¿³  __               ³
 Coil ³³  dial ³³ ³  ³  PP3 Battery ³
  (  )³³  ( )  ³³ ³VU³              ³__  BNC to
      ³³Dip Osc³³ ³__³  [] on/off   ³__³ Counter
      ³ÀÄÄÄÄÄÄÄÙ³                   ³
       ~~~~~~~~~~~~~~~~~~~~~~~~

In practice some of the tin plate is left off for battery access etc.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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